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Skin Deformation Methods for InteractiveCharacter Animation
Nadine Abu Rumman1(B) and Marco Fratarcangeli2
1 Department of Computer, Control, and Management Engineering,Sapienza University of Rome, Via Ariosto 25, 00185 Rome, Italy
[email protected] Department of Applied Information Technology,
Chalmers University of Technology, Lindholmsplatsen 1,41296 Gothenburg, Sweden
http://www.dis.uniroma1.it/~aburumman/
http://www.fratarcangeli.net/
Abstract. Character animation is a vital component of contemporarycomputer games, animated feature films and virtual reality applications.The problem of creating appealing character animation can best bedescribed by the title of the animation bible: “The Illusion of Life”.The focus is not on completing a given motion task, but more impor-tantly on how this motion task is performed by the character. This doesnot necessarily require realistic behavior, but behavior that is believ-able. This of course includes the skin deformations when the characteris moving. In this paper, we focus on the existing research in the areaof skin deformation, ranging from skeleton-based deformation and vol-ume preserving techniques to physically based skinning methods. Wealso summarize the recent contributions in deformable and soft bodysimulations for articulated characters, and discuss various geometric andexample-based approaches.
Keywords: Character animation · Deformation · Skinning · Skeleton-based animation · Volume-preserving · Physics-based animation
1 Introduction
Producing believable and compelling skin deformations for articulated charactersis a multi-disciplinary problem, which can be divided into three main problems:generating high-quality skin deformations, simulating skin contact in responseto collisions, and producing secondary motion effects such as flesh jiggling whena character moves. Traditionally, bone transformations describe the position andorientation of the joints, and the skin deformation is computed by linearly blend-ing bone transformations to the skin. However, such simple and linear blending tothe bone transformations cannot be expected to capture complex deformations.In contrast, by employing a physically based method into the skinning process,c© Springer International Publishing AG 2017J. Braz et al. (Eds.): VISIGRAPP 2016, CCIS 693, pp. 153–174, 2017.DOI: 10.1007/978-3-319-64870-5 8
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the believability of character motions is highly enhanced. Physics-based simu-lations manage to bring skeleton-based animation beyond the purely kinematicapproach by simulating secondary motions like jiggling, volume preservationand contact deformation effects. Despite offering such realistic effects, physicallybased simulation is computationally demanding and complex, thus it is usuallyavoided in interactive applications.
The key challenge of producing believable deformations is to satisfy theconflicting requirements of real-time interactivity and believability. Believabil-ity requires achieving sufficient deformation detail, which means capturing thefull range of desired effects. Producing these deformations demand at least anorder of magnitude more computation time than current interactive systems.The aim of this paper is to provide a comprehensive survey on the existingskinning techniques in literature. These techniques can be classified into threecategories: skeleton-based deformation (geometry and example-based skinning),volume preserving skinning and physics-based skinning methods. We discuss theexisting skinning methods and how they address the above-mentioned problems,and we also highlight the advantages and disadvantages of each method.
2 Skeleton-Based Deformations
The most common approach for deforming articulated character’s skin is todefine the surface geometry as a function of an underlying skeletal structure.Due to the simplicity, intuitive manipulation, and the ability to quickly solveinverse-kinematics on a small subspace (i.e., the skeleton), skeleton-based meth-ods are very popular and widely used in the animation industry. When modelinga skeleton-based deformation, the challenge is to obtain high-quality skin defor-mations. The current skeleton-based deformation techniques can be divided intotwo sections: geometry-based methods (also called smooth skinning, Sect. 2.1)and example-based methods (Sect. 2.2).
2.1 Geometric Skinning Techniques
In geometric skinning techniques, skeleton-to-skin binding is defined in a direct,geometrical way. Geometric approaches to deform articulated characters haveshown reasonable results at interactive rates. We start by discussing the stan-dard real-time method “skeletal subspace deformation”, also known as linearblend skinning (LBS). This method has been widely adopted in real-time appli-cations such as games, for its computational efficiency and straightforward GPUimplementation.
Linear Skinning Methods. The fundamental technique to drive the defor-mation of a character skin is via an underlying skeleton. Among the manyproposed skeleton-based deformation techniques, linear blend skinning (LBS)is the most popular technique due to its effectiveness, simplicity, and efficiency
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[57]. It has been given many different names over the years, including “skele-tal subspace deformation”, matrix palette skinning, enveloping, vertex blending,smooth skinning (Autodesk Maya), bones skinning (Autodesk 3D Studio Max),or linear blend skinning (the open-source Blender). What follows is an explana-tion of both the concepts and mathematics necessary to understand linear blendskinning. We will also review the algorithm and its limitations. In linear blendskinning, the basic operation is to deform the skin according to a given list ofbone transformations (Fig. 1). The formulation of the LBS model requires thefollowing input data:
Fig. 1. An example illustrates the main concept of LBS. There are two transformationsT1 and T2, corresponding to the transformations of shoulder and elbow joints fromthe rest pose to an animated posture.
– Surface mesh, a 3D model represented as a polygon mesh, where only vertexpositions will change during deformations. We denote the rest pose verticesas vi, . . . ,vn, and vi ∈ R
4 is given in homogeneous coordinates.– Bone transformations, representing the current deformation using a list of
matrices T1, . . . ,Tm ∈ R4×4. The matrices can be conveniently defined using
an animated skeleton. Tj is the transformation matrix associated with bonej in its current (animated) pose, R−1
j is the inverse transformation of thesame bone in the rest pose R. Additionally, the matrix R remains constant,so its inverse is also constant and can safely be pre-computed. Each bonetransformation influences part of the mesh.
– Skinning weights, For each vertex vi, we have weights wi,1+ . . .+wi,m ∈ R,in which each wi,j describes the amount of influence of bone j on vertex vi.The binding weights are normally assumed to be convex, so wi,1+. . .+wi,m =1 and wi,j ≥ 0.
The surface mesh is driven by a set of bones. Every vertex is associated withthe bones via a bone-vertex bind weight, which quantifies the influence of eachbone to the vertices. Therefore, the basic idea behind LBS is to linearly blendthe transformation matrices. The skin is deformed by transforming each vertexthrough a weighted combination of bone transformations from the rest pose.Thus, the final transformed vertex position v′
i is a weighted average of its initialposition transformed by each of the attached bones. The whole process can beexpressed with the following equation:
156 N.A. Rumman and M. Fratarcangeli
v′i =
m∑
j=1
wi,jTjR−1j vi (1)
In practice, it is rarely makes sense to attach a vertex to more than fourjoints. However, some old games systems used a variant dubbed rigid binding,which corresponds to allowing only one influencing bone per vertex. Rigid bindingleads to unrealistic non-smooth deformations near joints and suffers from self-intersections. With increasing polygon budgets, linear blend skinning quicklyreplaced rigid binding because it allowed for smooth transitions between indi-vidual transformations (see Fig. 2). Linear blend skinning begins by assigningweights for every vertex on the skin mesh to the underlying bones. The bind-ing weights can be constructed automatically based on the distance betweenthe skin vertices and line segments representing the bones, but it is hard toreliably create good weights automatically. Usually the artists must paint theweights on the mesh directly, using their knowledge of anatomy. Technically, theimplementation of a linear blend skinning algorithm is straightforward.
Fig. 2. Left. Linear blend skinning. Note the loss of volume at the elbow joint. Right.Rigid binding. Note the self-intersections and unnatural deformations in the areasaround the elbow joint.
Linear blend skinning works very well when the blended transformations Tj
are not very different. Issues arise if we need to blend transformations, which dif-fer significantly in their rotation components. Despite its fast and straightforwardimplementation, linear blend skinning suffers from some visible artefacts whenwe rotate joint more than 90◦. In a rotating joint, we expect the skin to rotatearound the joint too, maintaining the volume. But the linear blend model insteadinterpolates skin vertex positions linearly between where the bones expect them,which shrinks the volume of the skin. Therefore, this linearly blending rotationleads to the well-known “candy-wrapper” artefact (as we can see in Fig. 3).
Figure 4 illustrates the problems of LBS, which are loss of volume when bend-ing and the “candy-wrapper” artefact when twisting. The limitations of LBS havebeen extensively studied, where many techniques have been proposed to avoidits artefacts. One possibility is to enrich the space of skinning weights, lead-ing to methods which are still linear but offer a wider range of deformations.These methods are called multi-linear skinning techniques [62,86], in which the
Skin Deformation Methods for Interactive Character Animation 157
Fig. 3. The well-known “candy-wrapper” artefact of linear blend skinning. Left. Thecharacter model in its rest pose. Right. The model deformed with linear blend skinning,where the areas around the shoulder joint suffer from the “candy-wrapper” artefact andvolume loss when twisting.
extra weights are learned from input examples and regularization is used toprevent overfitting. Merry et al. propose a multi-linear skinning model calledAnimation Space [59], which uses 4 weights per vertex-bone pair. However, thisincrease in the number of weights carries an additional cost in time and space, aswell as parameter passing. While linear skinning techniques are popular due totheir efficient implementations, they cannot entirely remove the “candy-wrapper”artefact, which is in all cases noticeable under large joint rotations. For a com-prehensive survey on linear skinning techniques, we refer the interested reader to[34]. Selecting good skinning weights is critical to avoid the artefacts and gener-ate more natural deformations. Recently, an automatic computation of skinningweights was presented in [16]. In their method, the influence weights are deter-mined using geodesic distances from each bone, which makes the inverse-distanceweights shape-aware and can even work with production meshes that may con-tain non-manifold geometry. Despite that associating skinning weights with themesh vertices can be done automatically, this method tends to either increaseor decrease the volume around joints.
Fig. 4. Left to right: The skin in its rest pose. Rigid transformations (express rotationand translation). While twisting, the weighted combination of vertices v1 and v2 is notguaranteed to be a rigid transformation, which result in the “candy-wrapper” artefact.When bending, the linear interpolation of LBS between the vertices v1 and v2 producesv at an inadequate location, which result in a loss of volume.
Nonlinear Skinning Methods. By replacing linear blending with nonlinearblending [29,42,45], the “candy-wrapper” artefact can be completely avoided.Nonlinear skinning methods convert the affine rigid transformation matrices
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to <quaternion, translation> pairs, which are then easier to blend than theirmatrix equivalents. In [29] some constraints are imposed on the character’s rig-ging, whereas spherical skinning [45] uses a computationally expensive SingularValue Decomposition (SVD) scheme. The practical impact of these two meth-ods is limited because of their dealing with the translational component of theskinning transformations. In contrast, dual quaternion skinning (DQS [42]) usesan approximate blending technique based on dual quaternions (essentially, tworegular quaternions). DQS delivers a rigid transformation in all cases and it isalmost as fast as LBS. While the underlying mathematics may not be trivial,an actual implementation of dual quaternion skinning is quite straightforward.Instead of using matrices to express the rigid transformations of the bones, dualquaternion skinning uses the geometric algebra of quaternions. The formulationof DQS is based on dual quaternions [13], which is a generalization of regularquaternions that can be used to express both the translation and the rotation.We are blending rigid transformations, however, the blended matrices M are nolonger rigid transformations under rotations, but general affine transformations(potentially containing scale and shear factors). Therefore, instead of blendingmatrices M =
∑mj=1 wi,jTjR−1
j . In DQS, we blend dual quaternions qj. Thefigure below illustrates the intuition of why DQS is better than LBS.
To compute the deformed position of a vertex with DQS. First, the transfor-mation matrices are converted to unit dual quaternions. Then, these unit dualquaternions are blended linearly, similarly to linear blend skinning. In otherwords, we blend dual quaternion transformation qj, weighted by wi,j . The resultis normalized with ‖∑m
j=1 wi,jqj‖ to produce the final dual quaternion used totransform a vertex from its rest pose to the deformed position. This guaranteesto represent only a rotation and translation. While linearity is lost (due to theprojection on unit dual quaternions), the resulting algorithm can be implementedvery efficiently. Dual quaternion skinning computes deformed vertex positionsaccording to the following formula (a more detailed explanation on dual quater-nion skinning can be found in [43]):
v′i = qjviqj
∗ (2)
where qj =∑m
j=1 wi,j qj
‖∑mj=1 wi,j qj‖ is a unit dual quaternion and qj
∗ is the conjugateof qj. Unlike matrices, by using dual quaternions there will not be any scalefactor that shrinks the mesh around the joints. Dual quaternion skinning thussuccessfully eliminates the “candy-wrapper” effect, but it reveals its own artefact,called joint-bulging artefact (as we can see in Fig. 5). Moreover, dual quaternionskinning has a number of limitations especially in a production environment,as dual quaternions are unable to represent non-rigid transformations, such asshearing. The undesired joint-bulging artefact of DQS requires artistic manualwork to be fixed. Because fixing these artefacts manually is a tedious process,automatic skinning techniques are becoming increasingly popular [5,12,35].
An interesting extension of linear blend skinning called spline-skinning comesfrom [18,19], which often produces better skinning deformations and suppresses(but not completely eliminates) the “candy-wrapper” artefact. Instead of using
Skin Deformation Methods for Interactive Character Animation 159
Fig. 5. A demonstration of the artefacts of linear blend skinning (LBS) and Dualquaternion skinning (DQS). Left. LBS suffers from volume loss while bending. Right.DQS successfully eliminates the “candy-wrapper” effect and preserve the volume of theskin, but produces the joint-bulging artefact while bending.
conventional matrix rotation, spline-skinning represents each bone of the skele-ton by a spline. Furthermore, an appealing extension of DQS that is successfullyapplied in a production setting (Disney’s Frozen), can be seen in [53]. For aextensive discussion on nonlinear skinning methods, we refer the reader to [41].Whilst all the above-mentioned methods fully define the surface positions basedon skeletal configuration, they cannot capture secondary motion effects and skincontact behavior in response to collision. Recently, more advanced geometricskinning methods were introduced to limit the artefacts of LBS, while keepingtheir simplicity. Kavan and Sorkine [44] developed a new skinning method basedon the concept of joint-based deformers, which avoids the artefacts of linearblend skinning as well as the bulging artefact of dual quaternion skinning. Morerecently technique proposed by Jacobson and Sorkine [36], where they expandedskinning to support bending, stretching and twisting by using a slight variationon the standard skinning equations. Another impressive skinning results can beobtained using the technique presented in [83], which generates visually plausibleskin deformations in real-time (see Fig. 6). Their method automatically capturescontact surfaces between skin parts, without requiring any collision detectionstep. Moreover, they extended their framework to handle local skin contacts andproduce the effect of skin elasticity (sliding effect) [84]. More recently, [48] pro-posed a post-processing method for dual quaternion skinning, which eliminatesthe joint-bulging artefacts and its suitable for real-time character animation.
In spite of improvements, skinning using geometric skinning techniquesremains purely kinematic, lacking of secondary motions effects like passive jig-gling motion of the fatty tissues or muscle bulging. In the next section, we presentthe most influential example-based skinning methods, which are able to alleviatethe limitations of geometric skinning method, and add dynamic effects to theskin.
2.2 Example-Based Skinning Methods
In contrast to geometric approaches, example-based skinning methods (alsoknown as data-driven techniques) have permitted more complex skinning effectssuch as muscle bulges and wrinkles, while also addressing the artefacts of linear
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Fig. 6. Dana model in a break-dance pose. From left to right, the model is deformedwith linear blend skinning, dual quaternion skinning and implicit skinning. Note thevisible loss of volume produced by LBS (left). Implicit skinning (right), however, gen-erates visually plausible skin deformations, which avoids the artefacts of linear blendskinning, as well as the bulging artefacts of dual quaternion skinning [83].
skinning techniques. These methods take as input a series of sculpted exampleposes and interpolate them to obtain the desired deformation. One of the firstexample-based methods is pose space deformation (PSD [54]), which uses a radialbasis function to interpolate correction vectors among the example poses. In posespace deformation method, pose space is a set of degrees of freedom of a charac-ter’s model, which vary between the example poses. A particular pose is a set ofparticular values of these degrees of freedom. Pose space deformation comprisesone family of approaches, in which example poses (or local frame corrections) areinterpolated as a function of a character pose. A more sophisticated extension ofPSD was presented in [79]. Their method interpolates an articulated characterusing example poses scattered in an abstract space. This abstract space consistsof dimensions describing global properties of the 3D character, such as age andgender, in addition to dimensions that are used to describe the configuration,such as the amount of bend at the elbow joint. Moreover, PSD was general-ized to support weight (weighted pose space deformation WPSD [50,70]), whichlargely reduces the number of required example poses. Although WPSD canhandle large-scale deformations well, it cannot provide detailed deformation andit requires more computation than the original pose space deformation (PSD).In these methods, the amount of memory grows with the number of trainingexamples, thus they are more popular in animated feature film (DreamWorksAnimation’s Shrek 2) than in real-time application. To tackle this problem, [49]proposed a method similar in spirit to PSD called EigenSkin. Instead of usingall the displacements for example poses, they used precomputed principal com-ponents of deformation influences on individual joints. The resulting algorithmleads to considerable memory savings and enables to transfer the computationsto the GPU. Despite the fact that pose space deformation methods are sim-ple to implement, they require tremendous effort from artists, as they have tocreate different poses by hand for a wide variety of examples. Another class ofexample-based methods, which is a direct generalization of LBS, but does notrequire data interpolation, is formed by methods such as single-weight enveloping(SWE [62]) and multi weight enveloping (MWE [86]). Single-weight enveloping
Skin Deformation Methods for Interactive Character Animation 161
estimated single weight per vertex with rigid character bones, with provisionsmade for adding additional bones. Multi-weight enveloping, however, is basedon a linear framework supporting multiple weights per vertex-bone, where itprovides better approximations than SWE, but at the cost of 12 weights pervertex-bone, instead of 1 weight per vertex-bone in SWE. This class of meth-ods allows a smaller number of poses to be used to generate a larger number ofdeformations, while introducing more weight parameters. Thus, these numerousparameters come at a cost of complicated computation of the weights.
Fig. 7. A set of example poses from an anatomically motivated arm model with bothbending and twisting at the elbow. The twisting and muscle bulges are enough toprevent LBS from approximating the examples well. The technique of [62] does better,but still differ from the given example poses. The model from [85] well-approximatethe examples poses. (Image taken from [85].)
As an alternative to using sculpted example poses; several example-basedapproaches use scanned or photographed data. Early work that uses 3D scannedposes of a human body in character skinning has been presented in [61]. Addi-tionally, the method in [2] creates a high quality posable upper body model fromrange scan data and markers. In their method, to learn the skinning model, theyobtain deformations corresponding to different poses by matching a subdivisionsurface template to the range data. Recently, more advanced example-basedtechniques have been effectively integrated with mesh deformation algorithmsto further improve the quality of skinning [32,77]. A rotational regression modelwas proposed in [85], which captures common skinning deformation such asmuscle bulging (as we can see in Fig. 7) and twisting, specifically in challengingregions such as the shoulders. Park and Hodgins also introduced an interest-ing technique that captures and synthesizes detailed skin deformations such asbulging and jiggling [66,67], when a character performs dynamic activities. Theyuse a very dense and large set of markers to capture the dynamic motions. Then,they employ a second-order skinning scheme followed by a radial basis functionof the residual errors to provide detailed skin deformations. While high-qualityskin deformations can be captured accurately using scanned data, marker-basedmotion capture systems typically have a time-consuming calibration process andhigh hardware cost.
Example-based skinning methods are attractive since they can provide richdetails from physical measurements and add realistic secondary deformation to
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the skeleton-based animations. Shi et al. presented an appealing method that isable to provide the jiggling of the fatty tissues in real-time by taking a surfacemesh and a few sample sequences of its physical behavior [77]. Moreover, themethod proposed in [32] is capable of synthesizing high resolution hand meshdeformation with rich and varying details, from only 14 examples poses. How-ever, these approaches do not capture detailed soft-tissue deformations on awide variety of body shapes. This limitation has been addressed by (Dyna [68]),which learns a model of soft-tissue deformations from examples using a high-resolution 4D capture system. Dyna captures surface deformations of the bodyat high spatial and temporal resolutions, and constructs a mathematical modelfor relating these deformations to the motion and body shapes of novel char-acters. The major drawback of example-based methods is the need for exampleposes. Besides the fact that when the example poses cannot be captured on areal actor, creating these poses requires either tremendous effort from an artist,or a complex physical simulation on a volumetric version of the skin mesh. Inboth cases, the mesh and its associated skeleton at rest are not sufficient, andfurther human intervention is required. An interesting discussion on example-based deformation methods, can be found in [17]. In the next section, we discussseveral volume preservation methods for skinned characters, which have beenproposed to tackle the loss of volume artefact of linear skinning techniques.
3 Volume Preserving Skinning Methods
Volume preservation is an important aspect in the context of skin deformationthat has been addressed in a variety of research papers over the last years. Vol-ume preservation methods allow artists to correct the volume changes throughthe generation of extra bulges and/or wrinkles. The method that has been pro-posed in [15] is one of the first methods to introduce volume preserving defor-mation, where they use local volume controllers to guarantee volume conserva-tion of implicitly described soft substances. Moreover, multi-resolution methods[7] can preserve surface details by decomposing a mesh into several frequencybands. Furthermore, Funck et al. presented an appealing approach that deformsthe mesh vertices based on vector field integration [20,21]. However, these twomethods are either computationally expensive or do not fit into the standardanimation pipeline. Angelidis and Singh developed a skinning algorithm basedon a powerful embedding into the volumetric space, which enables to preservevolume locally and globally [3]. In their method, a degree of freedom is left to theartist to control the final shape, although its combination with skinning weightsvariation along the mesh makes this control somewhat indirect. Recently, [71]presented an automatic volume correction method to model the constant volumebehavior of soft tissues. It corrects the resultant deformations of LBS using aset of local deformations. In their work, they used an automatic way to segmentan organic shape into a set of regions corresponding to the main muscle andfatty tissue areas, in which volume is computed and locally corrected. Huanget al. employed a nonlinear version of the volumetric graph Laplacian, which
Skin Deformation Methods for Interactive Character Animation 163
features nonlinear volume preservation constraints [33]. Lipman et al. introducea shape and volume preserving mesh editing technique [55], where meshes arerepresented by moving frames. These frames are scaled during deformation suchthat the volumetric shape properties are preserved.
Several impressive works that create an inner scaffolding of spring, whichresist compression to maintain volume are described in [30,88]. The method in[88] provides an excellent introduction to these interior lattice methods. Lattice-Based freeform deformation (FFD) are widely-used in commercial software (suchas Autodesk 3D Studio Max and Maya) for providing smooth deformations andpreserving the volume of the skin [60]. For example, Autodesk Maya 2007 sup-ports the notion of flexors. This lattice flexor uses a local FFD lattice, whichcan then be driven by joint transformations. However, the flexors do not sup-port skinning transfer and the use of flexors can require significant setup andtweaking because of the multitude of lattice points. FFD was first formally pro-posed in [76] both as a representation for free-form solids and as a method forsculpturing solid models. Using FFD, a complex character can be deformed bypositioning the control vertices of the coarse control grid. A more general exten-sion of FFD (EFFD) was later presented by [14]. Moreover, Hsu et al. provideda method that directly manipulates the FFDs [31] and the method in [4] usesan independent deformation function to provide a more flexible FFD. Althoughlattice-based methods give the artist the flexibility of creating the desired defor-mation, they require additional setup work and the deformation is sometimesdifficult to predict.
On the other hand, cage-based skinning techniques consider an appealingway to control the deformation of an enclosed fine-detailed mesh and help topreserve the volume of skin deformations [39,75]. Cage-based techniques can beconsidered as a generalization of the lattice-based freeform deformation. Insteadof a regular control lattice, a cage is defined by a fixed-topology control latticethat is fitted to the character skin. The cage can be seen as a low-resolutionabstraction of the character, which enables the user to deform a character usinga simpler mesh. Most cage-based deformation methods are special case of linearblend skinning, where the handle (cage vertex) transformations are restricted tobe translations and the focus is on choosing the weights. The method presentedin [40] uses cage-based deformations to implement skinning templates, whichoffer a flexible design space within which to develop reusable skinning behavior.In their method, the skeleton drives the motion of the cage vertices using anexample-based skinning technique, where the cage smoothly deforms the char-acter model (see Fig. 8). Joshi et al. proposed a powerful cage-based deformationmethod based on harmonic coordinates [38] for use in high-end character artic-ulation. Their technique guarantees that the influence of each cage vertex isnon-negative and falls off with distance as measured within the cage. It gen-erates a pleasing deformation, but computing the harmonic coordinates is noteasy. In spite of that, cage-based techniques allow smooth deformation of skingeometry. Posing the cage requires manual manipulation of the cage vertices. Foran overview discussion on volume-preserving deformation methods, we refer the
164 N.A. Rumman and M. Fratarcangeli
reader to [22,65]. Another promising way to preserve the volume of the skin andto achieve realistic deformation is by applying physics-based simulation into theskin layer around the skeleton. The following section describes the vast literatureon physics-based methods.
Fig. 8. Skinning with cage: (a) Input geometry with skeleton. (b) An initial cage con-structed from four templates, which are associated with the hand joint, elbow joint,upper arm bone, and the shoulder joint. (c) The skeleton deforms the mesh templates.(d) The geometry is deformed by the cage, yielding a non-pinching elbow and musclebulging. (Image taken from [40].)
4 Physics-Based Methods
In order to model dynamic phenomena, such as the vibration of fatty tissues,muscles bulging and skin contact deformations due to collisions, the animatormust configure the deformation for each keyframe. While manually posing a char-acter for each animation keyframe allows artists to create such realistic effects,this process is tedious. Therefore, an alternative method is to employ physicsinto the skinning process, which highly enhances the believability and realismof character motions. Accordingly, physics-based simulation manages to bringskeleton-driven animation beyond the purely kinematic approach by simulatingsecondary motions, such as jiggling of soft tissues when the character is mov-ing. Those secondary motions enrich the visual experience of the animation andare essential for creating appealing characters for movie productions and virtualreality applications. After the pioneering work of Terzopoulos et al. [81] andconcurrently Lasseter’s animation principle “squash and stretch” [52]. Physicalsimulation has taken an important role in the animated feature film industryand computer games, where many physically based methods encouraged to sim-ulate soft bodies or add dynamic effects to the skin. In the following subsections,we first discuss soft body simulations in (Sect. 4.1), and then physically basedskinning methods in (Sect. 4.2).
4.1 Deformable and Soft Bodies Simulations
Simulating soft bodies can be achieved in different ways, and the design choiceoften has to balance the required accuracy and performance. The most popu-lar techniques for simulating soft bodies in computer animation are force-basedmethods. In particular, most of the techniques used to simulate dynamics rely on
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mass spring systems, because of the simplicity and efficiency. The general idea isto represent the vertices of the mesh as mass points, governed by Newton’s secondlaw of motion, and the edges as elastic massless links (spring). Hence, the meshis deformed when the lengths of the elastic links change. This happens when therelative position of the mass points changes due to external forces. Mass-springsystems are based on a local description of the material, in which the physics ofsuch systems is straightforward and the simulator is easy to implement. How-ever, to simulate a particular material, it is important to select carefully theparameters of the springs, such as the stiffness and damping. Despite that thesesystems are fairly easy to implement, they suffer from instability and overshoot-ing problems under large time steps. Moreover, mass-spring systems are oftennot accurate, since they are strongly topology dependent and are not built basedon elasticity theory. On the other hand, finite element methods (FEM) allowsto model elastic materials, in which both the masses and the internal and exter-nal forces are lumped to the vertices. The vertices in the mesh are treated likemass points in a mass spring system while each element acts like a generalizedspring connecting all adjacent mass points. The methods presented in [11,81] arethe first to demonstrate the effectiveness of comparatively simple mass-springbased approaches. In their methods, they applied the Lagrangian equations ofmotion using a finite difference scheme to simulate elastic objects with regularparametrizations. Here, the physical material properties can be described usingonly few parameters that are used to model soft bodies in an accurate man-ner. Unlike mass-spring systems, finite element methods are easy to simulate forany particular material. This makes things easier for artists in charge of mod-elling different types of soft bodies. Unfortunately, finite element methods areavoided in real-time applications, because they are computationally expensiveand complex to implement. Various methods have been proposed to addressthe drawbacks of mass-spring systems and finite element methods [26,51,69].A comprehensive survey of Nealen et al. [64] provides the details about thesetechniques. The concept of employing dynamic simulations into skinning for thepurpose of character animation was introduced over two decades ago [25], wheremany techniques were proposed to reduce the accuracy of the simulation to helpimprove performance and interactivity. Capell et al. [10] used a volumetric finiteelement mesh to represent the deformation of skin, driven by the underlyingskeleton motion. They extended their method to include rigging forces, whichguide the deformation to a desired shape [9]. In their method, they effectivelyhandled the effect of skin movement by using skeletal constraints, but by usingforces that can violate the conservation of momentum makes their simulationunstable under large time steps. Shinar et al. [78] presented a framework ofa two-way coupling between rigid and deformable bodies, in which they use atime integration scheme for solving dynamic elastic deformations in soft bod-ies interacting with rigid bodies. However, their method does not facilitate thedevelopment of an interactive animation system, because of the massive com-putation required for the finite elements representing the deformable body. Incontrast, a possible way to accelerate the simulation of soft bodies is to focus
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on the surface rather than the volume [8]. In particular, Galoppo et al. [23] pre-sented a fast method to compute the skin deformation on the surface of a softbody with rigid core. Their formulation only considers the elastic energy fromskin-layer deformation, and does not include the deformation inside the volume.This may lead to inaccuracies when capturing pose-dependent deformations. Allthe above methods are only valid for small deformations and are unsuitable foran articulated character’s large deformations. On the other hand, Muller et al.[63] achieved a good real-time performances for large rotational deformations,by using a pre-computed linear stiffness matrix to generate the deformations;their method is simple and rotationally invariant. Recently, Kim and Pollard[46] proposed an approach relying on finite element method to simulate theskin deformation, able to handle both one-way and two-way simulations. Theirmethod generates compelling dynamic effects and the deformations are obtainedat near interactive rate. Jain and Liu presented a robust approach that realisti-cally simulate characters with soft tissue at the site of contact, where they usedtwo-way coupling between articulated rigid bodies and deformable objects [37].Liu et al. developed a framework that simulates and controls skeleton driven softbody characters [56]. Their method couples the skeleton dynamics and the softbody dynamics to enable two way interactions between the skeleton, the skingeometry, and the environment at interactive rates (as we can see in Fig. 9).
Fig. 9. Employing dynamic simulation into skinning process allows two-way interac-tions between the skeleton, the skin geometry, and the environment at interactive rates.(Image taken from [56].)
4.2 Physically Based Skinning
Physics-based methods are the natural choice for creating secondary motioneffects such as flesh jiggling when a character is moving [24,82]. Turner andThalmann model the elasticity of skin for character animation and simulate thefat layer by Hookian spring forces [82]. However, they treat muscles as purelycontrolled elements. Thus, they do not model muscles with deformable methods.Wilhelms [87] presented an approach for animated animals by simulating indi-vidual bones, muscles, soft tissues and skin. The use of muscles, soft tissues andflesh elements makes it hard to fit this approach into the skinning framework.Moreover, Hahn et al. [27,28] generated secondary skin dynamics based on therig degrees of freedom. Their methods simulate the deformation of a character’sfat and muscles in the nonlinear subspace induced by the character rig.
Skin Deformation Methods for Interactive Character Animation 167
Fig. 10. The Method of [58] takes a skeleton and a surface mesh as input. Based ona hexahedral lattice with 106,567 cells (center), their method simulates the deformedsurface (right) obeying self-collision and volumetric elasticity at 5.5 s per frame. (Imagetaken from [58].)
In the other direction, Kim and James [47] proposed a domain-decompositionmethod to simulate articulated deformable characters entirely within a subspaceframework, where they combined locally rotated nonlinear subspace models tosimulate the detailed deformations of the models. In order to simulate the mus-culotendons of the human hand and forearm, [80] add anatomic detail using thetendons and bones. While physics-based skinning methods can automaticallygenerate secondary motion with high visual quality, they entail a significantcomputational burden that slows production and prohibits its use in interactiveenvironments. McAdams et al. [58] presented a robust method using a uniformhexahedral lattice, which provides convincing deformations of the skin with con-tact handling. In addition, they introduce a one-point quadrature scheme anda multi-grid solver in order to improve the performance and stabilize the sim-ulation. Although their method can capture appealing skin deformations andguarantee pinch-free geometry, it works at best at near interactive performance(as we can see in Fig. 10). Recently, Bender et al. [6] introduced a multi-layercharacter skinning based on shape matching with oriented particles, used tosimulate the elastic behavior of a closed triangular mesh as a representation ofa skin model. They make a use of position-based constraints for coupling theskeleton with the skin and handling self-collisions. In [1,72–74], a two-layeredapproach, the skin is first deformed with a classic linear blend skinning and thenthe vertex positions are adjusted using Position-based Dynamics. This allows torobustly mimic the behavior of the skin, and achieve and tune effects like volumepreservation and jiggling at interactive rate.
5 Conclusion
In this paper, we have reviewed the common techniques for modelling defor-mations, especially those for character animation purposes. A greater attentionwas paid to skeleton-based methods, and physics-based methods. In skeleton-based skinning such as linear blend skinning, dual quaternion skinning and posespace deformation, surface deformation is restricted to the skeletal pose thatfully defines the surface deformation. While skeleton-based methods can pro-duce good results, the believability of the deformation using these methods is
168 N.A. Rumman and M. Fratarcangeli
limited. Because they cannot capture secondary motions effects and skin con-tact deformations. Skeleton-based deformation alone is not sufficient for cap-turing believable skin deformations, such as skin stretching, secondary motioneffects and skin contact due to collisions. In contrast, physics-based simulationsbring skeleton-based animation beyond the purely kinematic approach by sim-ulating secondary motions such as jiggling of soft tissues when the character ismoving, as well as capturing skin contact deformation. Simulating such flesh-like deformations is difficult due to the coupling between the skeleton and softskin. Moreover, the resultant deformation has a high number of independentdegrees of freedom, in which it does not respect any manipulation done by theartist. Therefore, once the deformation parameters are specified, it is difficultto control the actual resulting shape of the character in every animation frame.Furthermore, physics-based methods are computationally expensive and usuallyavoided in interactive applications. In the future skinning research, the com-putational process for obtaining believable skin motion must trade-off betweenthese requirements: it must (1) be fast enough to achieve interactive rate (i.e.,>30 fps), (2) produce believable animation to minimize manual post-processingtime, and (3) be controllable and stable.
Acknowledgements. The authors would like to express their deepest thanks toDr. Bart de Keijzer for the careful proof-reading of this paper.
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